The primary purpose of such arrangements is to permit the necessary air exchange between the water containing flow space of the system and the outside during filling and emptying of the system and when thermally caused volume changes take place in circulating water. Further, such arrangements are also intended to collect and separate air residues, which have remained in the system, and other gases which have been formed during the operation of the system, which air residues and other gases are carried along by the circulating water in the form of bubbles of different sizes.
Prior art arrangements of this kind are sometimes referred to as "de-aerators" and comprise a valve which is arranged at the upper end of the air collecting vessel which customarily is in the form of an upright, usually cylindrical, vent tank or receptacle. The closure body of this valve is in these constructions normally held in closing position against the outer air by means of a spring or against a force of a spring. When the water in the air collecting vessel drops below a predetermined level, the closure body of the valve is opened by a float, which is arranged in the vessel and connected to the valve. In order to connect the vent casing of the arrangement which forms the air collecting vessel, with the flow conduit of the system, a threaded pipe or nipple is arranged at the outer face of the casing bottom. The threaded nipple is screwed into a complementary threaded bore of a horizontally extending conduit section of the circulatory system. The inner cross-section of this threaded pipe or nipple is not only substantially smaller than the cross-section of the air collecting vessel but is necessarily also substantially smaller than the inner cross-section of that portion of the circulatory system which is to be connected with the atmosphere through the valve. The threaded pipes or nipples are for this reason on the exterior always provided with pipe threads of "1/8" and only in exceptional cases with larger threads of at the most "3/8" . This means that the flow cross section of the threaded pipes is always considerably below 1 cm.sup.2 while, customarily the flow cross section amounts to but several mm.sup.2.
Such prior art de-aerators generally fulfill their intended purpose satisfactorily during filling and emptying of the system, as well as during larger volume changes of the circulating water, by permitting an unobstructed exchange of air between the flow space of the system and the ambient or outer air, provided they are arranged at the highest point of the system. If they are thus arranged, the de-aerators serve at the same time as level controllers for the entire system if the system is connected to a supply container which holds pressurized water (accumulator) and maintains the volume of the water circulating through the system constant. During the operation of the system such de-aerator permit, however, the discharge of air and other gases which circulate with the operating water, only in respect of that air and those gases which have been effectively separated from the operating water and have been able to collect in the casing of the device above the water level, to wit, in the air collecting vessel or space.
Experience and extensive tests have, however, confirmed that successful operation of such de-aerators of the kind previously indicated, is at best possible for a relatively short operational period only, to wit, for the period during which the operating water maintains the characteristics of freshly filled tap water. The chemicals which are often added to the circulating water in heating systems and which are intended to dissolve calcium salts and to inhibit corrosion, significantly change the viscosity and surface tension of the water to such an extent that air and other gases which rise within the water, can no longer exit or penetrate through the surface of the water in unobstructed manner. The air and gases rather form bubbles at the water surface which in many instances are encased by a liquid skin which can be destroyed or ruptured with great difficulty only. This phenomenon increases the more extraneous substances are gradually taken up by the water from the conduit walls and their connecting or junction areas. The water then increasingly assumes the characteristics of an emulsion or colloidal liquid. This in turn results in the fact that below the apex of horizontal conduit pipes, bubbles, which are carried along by the relatively rapidly flowing water, pass by the junction areas of the prior art de-aerators without any exchange taking place between the gas contained in the bubbles and the water contained in the casing of the de-aerating device. This phenomenon increases and is aggravated after some time, since during passage of the operating water through the pump, bubbles, which initially were of relatively large size, are increasingly divided and thus made smaller and thereby better distributed within the water body, until a more or less viscous foam is formed which again does not reach the casing or air collecting space of the device. As a matter of fact, and as is well known from experience, larger foam concentrations, which are carried along by the circulating water, may cause serious operational disturbances. Thus, for example, when the pump rotor rotates within the foam concentrations, the speed of the rotor will suddenly increase with the result that the pump motor runs out of control. The pump then stops the water circulation and ultimately, due to dry run of the water lubricated bearings, both pump and motor are destroyed. This danger is particularly great in modern apartment heating systems, in which the pump oftentimes is arranged at the highest point of the circulatory system, a point which is particularly favorable for the collection of air in the pump space.